Mechanism and engineering of bacterial 1,3-1,4-&B.-glucanases: From glucan hydrolase to glycosynthases in enzymatic oligosaccharide synthesis

Bacillus 1, 3-1, 4-β-glucanases are retaining endo-glycosidases of family 16 GH. The hydrolyticmechanism of the Bacillus licheniformis enzyme follows a double-displacement reaction involving a catalytictriad with G1u134 as enzyme nucleophile, G1u138 as general acid-base, and a third residue, Asp136, that assistsin catalysis. It is shown that the carboxylate side chain of Asp136 participates in both glycosylation and deglycosylation steps contributing to the pKa modulation of the general acid-base during the enzyme cycle. Upon mutation of the catalytic nucleophile to alanine, the B. licheniformis 1, 3-1, 4-β-glucanase is a highly efficientendo - glycosynthase, with strict specificity for formation of 11-1, 4 glycosidic bonds. It is shown that Gin 138 acts as a general base in the transglycosylation mechanism. Strategies to control donor-acceptor condensation, donor self condensation, elongation and polymerization reactions have been devised, and examples are here shown for their application in oligosaccharide assembly or polysaccharide synthesis.